1. Field of the Invention
This invention generally relates to a delay measurement apparatus and, more particularly, to a group delay time measurement apparatus with an automatic aperture value setting function.
This invention broadly relates to an apparatus for measuring a group delay time (to be merely referred to as a delay hereinafter where it is not misleading) of a general electric circuit network.
In particular, a phase gradient scheme is conventionally known as a scheme to use two signals having different frequencies and to measure a group delay time from a phase difference in the two frequency outputs. In group delay time measurement based on the phase gradient scheme, a frequency difference (which is called an aperture value) of two signals having different frequencies is set at the optimum value in accordance with an object to be measured, e.g., an electric circuit network, an electric component, or the like. This invention relates to a group delay time measurement apparatus (or merely a delay apparatus) with an automatic aperture value setting function which can automatically set the aperture value at the optimum value.
2. Description of the Related Art
As a means for measuring a group delay time of an electric circuit network, the Nyquist scheme that uses AM or FM waves, the phase gradient scheme that uses a sweep frequency signal and differentiates the phase characteristics of the sweep frequency signal, the phase gradient scheme that uses two signals having different frequencies and performs measurement from a phase difference between the two signals, and so on are available.
FIG. 1 is a block diagram of an example of a measurement apparatus based on the phase gradient scheme and disclosed in Japanese Patent Publication No. 58-10711 as a prior art. As shown in FIG. 1, a signal output from oscillator 1 is split by power splitter 2 into two paths. One of the split signals is input to phase detector 4 through object 3 to be measured such as an electric circuit network, while the other of the split signals is directly input to phase detector 4. Detector 4 detects the phase difference between the two input signals. A detected phase difference signal is converted into a digital signal by A/D (analog/digital) converter 5 and is output. When measurement is completed, oscillator 1 outputs a signal having a frequency different from that of the signal originally output by oscillator 1 under the control of controller 6. The output signal is supplied to A/D converter 5, thus obtaining a digital phase difference signal. Assuming that the voltages of the signals measured in the above manner are V1 and V2, and that the frequency difference of the signals output from oscillator 1 is .DELTA.f, group delay time D is indicated as follows: EQU D.varies.(V2-V1)/.DELTA.f
Note that D=d.phi./d.omega. where .phi. is the signal phase shift between .DELTA.f and .omega. is the angular frequency. As a result, the apparatus having the above arrangement can easily obtain group delay time from a phase shift component of object 3 at different frequencies.
However, such a conventional apparatus has the following problems. More specifically, the detection range of phase detector 4 is generally -.pi. to +.pi.. Therefore, when a phase shift component falls outside this range, it must be corrected as shown in FIG. 2. When measured values are located at, e.g., two sides of +.pi., i.e., are discontinuous as shown in FIG. 2, the calculation for obtaining group delay time D becomes very complicated. Even when the precision of the output voltage with respect to the phase difference of phase detector 4 is improved to have an error falling only within a range of .+-.1%, if measurement is performed for the discontinuous points, the resultant precision has an error falling within a range of (2.pi..times.1) %, thus increasing the error. Although the precision having an error of (2.pi..times.1) % is very high in the conventional apparatus, it is insufficient as a precision of a group delay time.
FIG. 3 shows a group delay measurement apparatus disclosed in Japanese Patent Publication No. 58-10711 to eliminate the drawbacks of the conventional apparatus described above.
Referring to FIG. 3, a signal output by oscillator 11 is split by power splitter 12 into two signals. One split signal is supplied to object 13 to be measured while the other split signal is supplied to one input terminal of phase detector 14. The signal supplied to object 13 is output to variable phase shifter 15 to be phase-controlled and then supplied to the other input terminal of phase detector 14. Detector 14 detects a phase difference between the two signals and outputs a detection signal. The detection signal is supplied to the signal input terminal of DC amplifier 16 serving as a phase controller for controlling the phase shifting of phase shifter 15. The detection signal is also converted into a digital signal by A/D converter 17, and the digital signal is applied to controller 18. Upon reception of the digital signal, controller 18 outputs a holding signal to DC amplifier 16 from output terminal 18a and a frequency control signal to oscillator 11 from output terminal 18b.
A signal having a frequency (f-.DELTA.f/2) is supplied to the apparatus having the above arrangement through oscillator 11. Phase detector 14 detects a phase difference between a signal obtained through object 13 and a signal from oscillator 11 and outputs a signal corresponding to the phase difference. The phase difference signal is negatively fed back to variable phase shifter 15 through DC amplifier 16. Phase shifter 15 controls the phase shift of the signal output from object 13 such that a phase difference between the signal output from object 13 and the signal supplied form oscillator 11 becomes zero. As a result, a zero signal is output from phase detector 14. The zero signal is supplied to controller 18 through A/D converter 17. Upon reception of the digital zero signal, controller 18 stops negative feedback to phase shifter 15 performed by amplifier 16 to hold the phase shift control value obtained by phase shifter 15, while it also supplies a signal to oscillator 11. Upon reception of the signal, oscillator 11 outputs a signal having a frequency (f+.DELTA.f/2), and phase detector 14 detects a phase difference between the signal output from oscillator 11 and the signal supplied through object 13. In this case, negative feedback by amplifier 16 is stopped in the manner as described above. Therefore, phase shifter 15 performs phase shift control of the signal supplied through object 13 at the phase shift control value set in advance for a frequency (f-.DELTA.f/2). Therefore, phase detector 14 outputs a signal corresponding to the phase shift of object 13 for a frequency change component EQU (f+.DELTA.f/2)-(f-.DELTA.f/2)=.DELTA.f
In this manner, with this apparatus, a phase difference detected by phase detector 14 for the frequency (f-.DELTA.f/2) is controlled to be zero, and under this condition a phase difference is detected for the frequency (f+.DELTA.f/2). Therefore, with this apparatus, since phase difference detection is performed constantly with respect to a phase difference of zero as a reference, the detection range can normally be .+-..pi.. Phase difference detection for discontinuous points is not performed unlike in the conventional case, and continuous detection can be constantly performed in the vicinity of the center of the detection range. The error in the measured signal depends only on the error of phase detector 14, e.g., 1%, and the measurement precision can be maintained as high as 1%.
When a delay of an object to be measured is to be measured by a network analyzer using a group delay measurement apparatus based on the phase gradient scheme described above, an aperture value is set. As is known, a delay is obtained by calculating a differential of a phase gradient of phase characteristics. More specifically, referring to the graph shown in FIG. 4, when the phases for two different frequencies f.sub.1 =f.sub.0 +.DELTA.F/2 and f.sub.2 =f.sub.0 -.DELTA.F/2 are set at .theta..sub.1 and .theta..sub.2, respectively, delay time .tau. can be obtained in accordance with the following equation: EQU .tau.=d.theta./d.omega.=.DELTA..theta./2.pi..multidot..DELTA.F=.DELTA..thet a./360.degree..multidot..DELTA.F (1)
where .theta. is a phase shift, .omega. is an angular frequency, .DELTA..theta. is a differential phase value, and .DELTA.F is a differential frequency value (aperture value).
As is apparent from equation (1), when differential phase value .DELTA..theta. is constant, the larger aperture value .DELTA.F, i.e., the wider the aperture, the narrower the measurement range of delay .tau.; the narrower the aperture (the smaller aperture value .DELTA.F), the wider the measurement range of delay .tau.. Thus, the delay measurement range is automatically determined in accordance with aperture value .DELTA.F.
In conventional delay measurement using, e.g., the network analyzer as described above, aperture value .DELTA.F is uniquely determined by a measurement frequency range set in the measuring device before measurement. More specifically, referring to FIG. 5, since number N of display pixels of display screen A in the horizontal direction is determined to be, e.g., 512, this number corresponds to number N of measurement points. Therefore, when measurement frequency range S set in the measuring device is determined, the frequency among measurement points f.sub.n-1, f.sub.n, f.sub.n+1, and f.sub.n+2 is uniquely determined at S/N.
In the conventional measuring device, the frequency indicated as S/N is directly used and set as aperture value .DELTA.F. In other words, when measurement frequency range S is determined, aperture value .DELTA.F=S/N is automatically set, and the delay range is accordingly set, as is apparent from equation (1).
However, with the conventional method, aperture value .DELTA.F is determined regardless of an object to be measured, and an optimum aperture for the object to be measured is not determined. This is because the values of delay differ depending on objects to be measured even when measurement is performed in the same frequency band. For example, in measurement of filter delay characteristics, one filter has a very high Q value like a quartz filter, and another filter has a very low Q value like an LCR filter. The phase gradients of their phase characteristics differ depending on the objects to be measured. When aperture value .DELTA.F is set to be narrow where the actual delay characteristics are as indicated by the broken line, as in FIG. 6. Thus, the mountain-like portions of the graph exceeding .pi. collapse and the resultant delay graph becomes as indicated by the solid line. When wide aperture value .DELTA.F is always set in order to eliminate this drawback, phase inversion occurs at .theta.=.pi. a wide dynamic range cannot be obtained. Therefore, wide aperture value .DELTA.F cannot always be set. In this manner, with the conventional method, an optimum aperture for an object to be measured is not set. With the aperture value set in accordance with the conventional method, some objects cannot be measured.